1. Field of the Invention
This invention relates to photographic systems having optical low-pass filters and, more particularly, to photographic systems having low-pass filters suited to dispersively obtain an image by using an image pickup tube, image pickup board, etc. in video cameras, electronic still cameras, etc.
2. Description of the Related Art
Video cameras using a solid state image pickup element of dispersive resolving cell structure have to generally optically perform space sampling when the output of an image is obtained. In this case, if the light from an object to be photographed includes components of higher spatial frequency than the sampling frequency, spurious signals depicting a texture and color shades that the object does not possess are generated. In other words, since frequency components which cannot be picked up by the photographic apparatus (or components of higher frequency than the Nyquist frequency), cannot be reproduced, a phenomenon called wave distortion (aliasing) arises, contributing to forming moire, false colors, and other defects in the picture taken.
For this reason, it has been the common practice in the art to use an optical low-pass filter arranged to constitute part of the photographic system so that the high spatial frequency components of the object are limited. Many such optical low-pass filters are in the form of a quartz plate or the like utilizing double refraction.
FIG. 6 is a diagram to explain a conventional optical low-pass filter 60 using the quartz plate to utilize double refraction. In the same figure, an entering light beam 61 is split into two parts, or an ordinary ray 62 and an extraordinary ray 63, to obtain the low-pass effect. Here, letting the distance between these split rays be denoted by D and the spatial frequency by f, the modulation transfer function, or so-called MTF(f), becomes a cosine function as follows: EQU MTF(f)=.vertline.cos .pi.Df.vertline. (1)
FIG. 7 is a graphic representation of this modulation transfer function MTF(f). As is understandable from this graph, the prescribed spatial frequency component included in the object can be controlled by controlling the thickness of the quartz plate.
However, the quartz plate is expensive and, moreover, the split light beam becomes a linearly polarized beam. In a case where it is desired to split it to, for example, two or more directions, the polarized state must be changed by using a phase plate. Therefore, the optical low-pass filter of this kind gets thicker. Hence, there is a drawback that the production cost becomes even higher.
On the other hand, Japanese Laid-Open Patent Application No. Sho 53-119063 proposes another optical low-pass filter which utilizes a phase type diffraction grating. FIG. 8 shows the profile of the optical low-pass filter proposed in this publication and is formed as a series of triangles by using acrylic resin.
This optical low-pass filter has its diffracting effect form, for example, a point image expanded in one direction with the intensity distribution shown in FIG. 9. By this, the low-pass effect is likewise obtained as with the quartz plate. FIG. 10 is a graphic representation of the modulation transfer function MTF(f) of the optical low-pass filter shown in FIG. 8.
Meanwhile, optical parts to be used in the video camera, electronic still camera, etc. are required in a general case to assure their optical performances in a relatively wide wavelength range near or at the visible light region.
The video camera, for example, is required to have its prescribed optical performance operate well in a range of wavelengths 400 nm to 720 nm.
Nonetheless, the optical low-pass filter shown in FIG. 8, because of its diffracting effect varying with wavelength, forms different point images with different wavelengths. That is, as shown in FIG. 10, the MTF characteristic changes depending on the wavelength.
For example, with the use of the diffraction grating of triangular profile shown in FIG. 8, when the design wavelength is taken at .lambda.=540 nm to obtain an MTF characteristic shown by the curve G of FIG. 10, a value of the spatial frequency for the wave length .lambda.=400 nm at which the MTF value becomes "0", i.e., the trap frequency f.sub.0 ', as shown by the curve B of FIG. 10, smaller than the trap frequency f.sub.0 for the curve G. Conversely, when .lambda.=720 nm, the MTF value never takes the value of "0" as shown by the curve R in the same figure, thus failing to have a trap frequency.
In such a manner, the low-pass filter of such a profile as shown in FIG. 8 differentiates the MTF value as the wavelength varies. Therefore, its resolving power is poor, and its use leads to the production of moire patterns and false colors.
For the low-pass filter of the diffraction grating type, besides the profile of a series of triangles, many other forms such as sine curve, or a series of trapezoids, have been attempted. U.S. Pat. Nos. 3,821,795 and 3,784,734 and Japanese Laid-Open Patent Application No. Sho 45-29614 disclose low-pass filters of trapezoid profile.
To realize the low-pass filter of trapezoid profile, there is a strong demand for improvement of the contrast in the low frequency band and reduction of the wavelength dependency.